1. Field of the Invention
The present invention relates to a combustion state detecting device for detecting a combustion state of an internal combustion engine by detection of a change in the quantity of ions which are produced at the time of burning the internal combustion engine, and more particularly to a combustion state detecting device for an internal combustion engine which is capable of preventing a high-voltage leakage caused by the disconnection of a secondary current path in the internal combustion engine with a low-voltage distribution.
2. Description of the Related Art
In general, in an internal combustion engine driven by a plurality of cylinders, the fuel-air mixture consisting of air and fuel introduced into the combustion chambers of the respective cylinders is compressed by moving up pistons, electric sparks are generated by applying an ignition high voltage to ignition plugs disposed within the combustion chambers, and an explosion force developed at the time of burning the fuel-air mixture is converted into a piston push-down force, to thereby extract the piston push-down force as an rotating output of the internal combustion engine.
There has been known that since molecules within the combustion chambers are ionized when the fuel-air mixture has been burned in the combustion chambers, ions having electric charges flow between the ignition plugs as an ion current upon application of a bias voltage to ion current detection electrodes (as usual, ignition plug electrodes are used) located within the combustion chambers.
Also, there has been known that the combustion state of the internal combustion engine can be detected by detection of a state in which the ion current occurs because the ion current is sensitively varied according to the combustion state within the combustion chambers.
FIG. 5 is a circuit structural diagram showing one example of a conventional combustion state detecting device for an internal combustion engine using a low-voltage distribution as disclosed in Japanese Patent Application Laid-open No. Hei 10-231770.
In the figure, an anode of a battery 1 mounted on a vehicle is connected to one end of a primary winding 2a of an ignition coil 2, whereas the other end of the primary winding 2a is connected to the ground through a power transistor 3 an emitter of which is grounded for interrupting the supply of a primary current.
A secondary winding 2b of the ignition coil 2 constitutes a transformer in cooperation with the primary winding 2a, and a high voltage side of the secondary winding 2b is connected to one end of the ignition plugs 4 of the respective cylinders (not shown) to output a high voltage of negative polarity at the time of controlling ignition.
Each of the ignition plugs 4 composed of counter electrodes is applied with the ignition high voltage to discharge and fire the fuel-air mixture within each of the cylinders.
The ignition coil 2 and the ignition plug 4 are disposed in parallel for each of the cylinders, however, in this example, only one pair of ignition coil 2 and ignition plug 4 are representatively shown.
A low voltage side of the secondary winding 2b is connected to an ion current detecting circuit 10 through a resistor 5 and a diode 6 which are connected in parallel and constitute current limiting means.
The resistor 5 suppresses a discharge current that flows from a capacitor C within the ion current detecting circuit 10 to the ignition plug 4 through the secondary winding 2b and suppresses a voltage developed at the high voltage side of the secondary winding 2b at the time of starting the supply of the current to the primary winding 2a.
The diode 6 is provided so that a direction of the secondary current (ignition current) I2 flowing at the time of applying the ignition high voltage becomes forward, and is arranged so as to suppress a potential difference between both ends of the resistor 5 at the time of controlling ignition.
The ion current detecting circuit 10 applies a bias voltage of a polarity opposite to the ignition polarity, that is, the positive polarity through the resistor 5 and the diode 6 which are connected in parallel and the secondary winding 2b detect an ion current corresponding to the quantity of ions generated at the time of burning.
The ion current detecting circuit 10 includes a capacitor C connected to the low voltage side of the secondary winding 2b through the resistor 5 and the diode 6 which are connected in parallel, a diode D disposed between the capacitor C and the ground, a resistor R connected in parallel with the diode D, and a Zener diode DZ for bias voltage limit which is connected in parallel with the capacitor c and the diode D.
A series circuit consisting of the capacitor C and the diode D and the Zener diode DZ connected in parallel with the series circuit are disposed between the low voltage side of the secondary winding 2b and the ground to constitute a charging path for charging the capacitor C with the bias voltage at the time of generating the ignition current.
The capacitor C is charged with the secondary current flowing therein through the ignition plug 4 which is discharged at a high voltage outputted from the secondary winding 2b when the power transistor 3 is off (when the current supplied to the primary winding 2a is interrupted). The charge voltage is limited to a predetermined bias voltage (for example, about several hundreds V) by the Zener diode DZ, and functions as bias means for ion current detection, that is, a power supply.
The resistor R within the ion current detecting circuit 10 converts an ion current flowing with the bias voltage into a voltage, and inputs the current to an ECU (electronic control unit) 20 as an ion current detection signal Ei.
The ECU 20 formed of a microcomputer judges the combustion state of the internal combustion engine on the basis of the ion current detection signal Ei, and conducts appropriate adaptive control so that no inconvenience occurs when it detects the deterioration of the combustion state.
Also, the ECU 20 arithmetically operates an ignition timing, etc., on the basis of travel conditions obtained from a variety of sensors (not shown) to output not only an ignition signal P to the power transistor 3 but also a fuel injection signal to an injector (not shown) for each of the cylinders and a drive signal to a variety of actuators (a throttle valve, an ISC valve, etc.).
FIG. 6 is an explanatory diagram showing a path of current flowing in the secondary winding 2b and the ion current detecting circuit 10 through the current limiting means, in which a path of a secondary current I2 flowing at a high voltage during the discharging operation of the ignition plug 4 (at the time of controlling ignition) is indicated by a solid line, whereas a path of an ion current i flowing at the bias voltage at the time of detecting the ion current is indicated by a dashed line.
Subsequently, the operation of the conventional combustion state detecting device for an internal combustion engine shown in FIG. 5 will be described with reference to FIG. 6.
As usual, the ECU 20 arithmetically operates the ignition timing, etc., in accordance with the travel conditions, and supplies the ignition signal P to the base of the power transistor 3 at a desired control timing to control the on/off operation of the power transistor 3.
As a result, the power transistor 3 interrupts the primary current flowing in the primary winding 2a of the ignition coil 2 to boost the primary voltage, and also develops the ignition high voltage (for example, several tens kV) at the high voltage side of the secondary winding 2b.
The secondary voltage is applied to the ignition plug 4 for each of the cylinders and allowed to generate a discharge spark within the combustion chamber to burn the fuel-air mixture. In this situation, if the combustion state is normal, a required quantity of ions are generated in the periphery of the ignition plug and within the combustion chamber.
Then, as described above, when the power transistor 3 is turned on in response to the ignition signal P, the current in the primary winding 2a starts to flow therein, to thereby develop the voltage of the positive polarity at the high voltage side of the secondary winding 2b.
In this situation, since the discharge current from the capacitor C to the low voltage side of the secondary winding 2b is limited by the resistor 5, the voltage developed at the secondary winding 2b is divided to the high voltage side and the low voltage side without being superimposed on the bias voltage.
At the time of starting the flow of a current in the primary winding 2a, even if the voltage of the positive polarity is developed at the high voltage side of the secondary winding 2b, since the discharge current from the capacitor C to the low voltage side of the secondary winding 2b is limited by the resistor 5 as described above, the voltage of the positive polarity developed at the high voltage side of the secondary winding 2b is suppressed so that there is no case in which the ignition plug 4 discharges.
Sequentially, at the time of interrupting the primary current, if the ignition high voltage is developed at the high voltage side of the secondary winding 2b to make the ignition plug 4 discharge, the secondary current I2 flows in the path (an arrow indicated by a solid line in FIG. 6) through the diode 6 to charge the capacitor C up to a predetermined voltage.
Also, since ions are generated by the discharge of the ignition plug 4, the ion current i flows in a path (an arrow indicated by a dashed line in FIG. 6) through the resistor 5.
In this way, with the diode 6 being connected in parallel with the current limit resistor 5, the secondary current I2 at the time of controlling ignition flows into the diode 6 without flowing in the resistor 5. Since this makes the potential difference between both ends of the resistor 5 drop, the ignition performance is improved.
Also, at the time of starting the flowing of the primary current, since the current limit function of the resistor 5 becomes effective, the discharge current from the capacitor C to the secondary winding 2b is limited to prevent mal-control and the drop of the bias voltage.
The conventional combustion state detecting device for an internal combustion engine thus structured suffers from problems stated below.
That is, in the case where disconnection occurs in the secondary current path, for example, when disconnection occurs at a position indicated by (A) in FIG. 5 or misfire occurs in the ignition plug 4, the voltage (its peak voltage is about 40 kV) developed at the secondary winding high voltage side vibrates as indicated by a broken line in FIG. 7, and the vibration of a voltage (its peak voltage is about 8 kV) which is different in amplitude from but synchronous with that of the high voltage side occurs even at the low voltage side. However, the vibrations appearing at the positive polarity side is limited by the bias voltage limit Zener diode DZ at the low voltage side so as to be suppressed to about 200 V or less.
Also, in the case where disconnection occurs at positions indicated by (B) and (C) in FIG. 5, although the capacitive discharge occurs in the ignition plug 4, discharge does not continue because the secondary current path is not formed with the result that operation is not normally made as the ignition device.
Accordingly, the conventional device suffers from such a problem that in the case where disconnection of the secondary current path or misfire in the ignition plug occurs, the high voltage is developed at the secondary winding low voltage side, so that it is leaked to the ion current detecting circuit, etc., to thereby damage the parts within that circuit, or because the secondary current path is not formed due to the disconnection of the secondary current path, discharge does not continue, as a result of which operation is not normally made as the ignition device.